The invention relates to an ink jet recording head in which a silicon single-crystal substrate is used for a spacer forming member, and a method of producing such an ink jet recording head.
An ink jet recording head has a pressurizing chamber formed by respectively attaching a nozzle plate in which nozzle openings are formed and an elastic plate to both faces of a spacer with an adhesive. The elastic plate is deformed by a piezoelectric vibrating element. Since the ink jet recording head of this type does not utilize a thermal energy as a driving source for ejecting ink drops, the ink quality is not thermally changed. Particularly, therefore, it is available to eject color inks which may easily be thermally deteriorated. In addition, an amount of displacement of the piezoelectric vibrating element can be adjusted so that the ink amount of each ink drop is desirably regulated. For these reasons, such a head is most suitably used for configuring a printer for color printing with a high quality.
When color printing with a higher quality is to be performed by using an ink jet recording head, higher resolution is required. As a result, sizes of a piezoelectric vibrating element, a partition wall of a spacer member, and the like are inevitably reduced so that higher precision is required in the steps of working and assembling such members.
Accordingly, it has been studied that members for an ink jet recording head are worked by adopting a parts-manufacturing technique utilizing anisotropic etching of a silicon single-crystal substrate in which minute shapes can be worked with high accuracy by a relatively easy method, i.e., a so-called micro machining technique. Various techniques and methods are proposed, for example, in Japanese Patent Application Laid-open Nos. Hei. 3-187755, Hei. 3-187756, Hei. 3-187757, Hei. 4-2790, Hei. 4-129745, and Hei. 5-62964.
When color images or characters are to be printed with a high quality, it is required not only to increase the arrangement density of nozzle openings, but also to perform the printing by a so-called area gradation in which the area of one dot is varied in accordance with an image signal. In order to perform such an area gradation, the ink amount of each ink drop in one ejecting operation must be reduced to be as small as possible, and high-speed driving must be enabled, thereby realizing a recording head by which one pixel can be printed by several ejections of ink drops.
To comply with this, first, the displacement amount of the piezoelectric vibrating element must be reduced, and the displacement must be instantaneously reflected as a volume change of a pressurizing chamber. In addition, in order to link the small volume change of the pressurizing chamber to the ejection of ink drops, it is necessary to reduce the pressure loss in the pressurizing chamber to a level as small as possible.
In order to efficiently link the displacement of the piezoelectric vibrating element to the volume change of the pressurizing chamber, it is essential to increase the rigidity of the pressurizing chamber. In order to reduce the pressure loss in the pressurizing chamber, it is essential to make the volume of the pressurizing chamber as small as possible.
In order to reduce the volume of the pressurizing chamber, it is first considered that the opening area of a spacer which forms the pressurizing chamber is reduced. In view of the working accuracy of the piezoelectric vibrating element which abuts against the spacer, the reduction is limited to about one arrangement pitch of the nozzle openings at the maximum. For this reason, the reduction of the volume must be realized by decreasing the depth of the pressurizing chamber.
In view of the handling of a spacer in the assembling step or the like, however, the spacer must have the rigidity of some extent. To comply with this, a silicon single-crystal having a thickness of at least 220 .mu.m must be used as a silicon single-crystal substrate which constitutes the spacer. If a thin substrate having a thickness less than 220 .mu.m, the rigidity is very low. This produces a problem in that damages or unpredictable warpage may disadvantageously occur in the assembling step.
As a method of forming a shallow pressurizing chamber in a sufficiently thick silicon single-crystal substrate by anisotropic etching, it may be contemplated to use a technique in which only one face of the silicon single-crystal substrate is etched, i.e., a so-called half etching method. Since the pressurizing chamber must be communicated with a nozzle opening for ejecting ink drops, it is necessary to form a through hole which elongates from the face where a nozzle plate is provided to the pressurizing chambers.
As well known in the art, in order to form a through hole H by anisotropic etching, as shown in FIG. 27, it is necessary to set an opening length so as to be about 1.7 (the square root of 3) or more times as large as the thickness of the silicon single-crystal substrate. If the employed substrate has a thickness of 200 .mu.m or more, the minimum length of the opening of the through hole is about 380 .mu.m.
As thus constructed, the volume of a communicating holes causes the volume of the pressurizing chamber to increase. In addition, the size of the communicating hole is equal to the thickness of the silicon single-crystal substrate, i.e., 220 .mu.m, and the length in the longitudinal direction is 380 .mu.m. Accordingly, there arises a problem in that the opening area of the silicon single-crystal substrate is increased and eventually the rigidity of the spacer is disadvantageously degraded.
In a recording head which uses a spacer made of a silicon single-crystal substrate, a piezoelectric vibrating element 130 of the longitudinal vibration mode is used as an actuator as shown in FIG. 28. The piezoelectric vibrating element 130 of the longitudinal vibration mode is fixed to a frame 135 together with a passage unit 134 which comprises an elastic plate 131, a spacer 132, and a nozzle plate 133, so as to be assembled in an ink jet recording head.
Distortion caused by a difference in coefficients of thermal expansion between ceramic constituting the piezolectric vibrating element 130 and a material constituting the frame 135, in general, plastic occurs substantially in a proportional manner to the length L of the piezoelectric vibrating element 130. When heat is applied in an adhering step so as to obtain a high adhesive strength and then the condition is returned to a normal use condition, a temperature difference of 40.degree. C. or more occurs. In the case where the effective length L of the piezoelectric vibrating element 130 is 5.5 mm, for example, an expansion difference of about 10 .mu.m is caused by the above-mentioned difference, so that the elastic plate 131 may be damaged. Although such a damage may not be caused, the passage unit having a relatively low rigidity is distorted by the stress caused by the difference in thermal expansion. As a result, there arises a problem in that the flying directions of ink drops go out of alignment and errors are caused in hitting positions, thereby degrading the printing quality.